As a light source for various light emitting devices such as a lighting device, a signal, a backlight for a liquid crystal display panel, etc., a light emitting device which emits a desired wavelength light like light emitting diode (LED) is used. Such light emitting diode, especially high brightness light emitting diode is brighter, when compared to a white color lighting device such as a filament lamp, mercury lamp, fluorescent bulb, etc., consumes less electricity and in addition has a long life. Therefore, it is used in many indoor or outdoor luminescent devices. In addition, in a solar cell assembly, photoelectric conversion element comprising P and N type silicones which performs photoelectric conversion is usually installed.
A wiring substrate on which a light emitting and absorbing element such as a light-emitting element and a photoelectric-conversion element is mounted or a package case that surrounds and accommodates these elements, is formed with a ceramic-made or resin-made reflecting substrate to reflect light from the light source in a direction to be irradiated or to reflect and collect light which enters a solar cell assembly toward a photoelectric-conversion element in order to carry out an energy conversion efficiently.
If the reflective substrate of the wiring base plate and the package case is made of ceramics, exiting light tends to leak, accordingly it is difficult to obtain the reflection efficiency.
As a resin-made reflective substrate, on the other hand, a base plate of a support body such as a sheet-like glass fiber is immersed into a resin composition containing an epoxy resin which contains an alicyclic epoxy resin, a glycidyl(meth)acrylate base polymer, a white pigment and a curing agent, as essential ingredients and then dried, to obtain a white prepreg (Japanese Patent Publication 2006-316173A).
Resin compositions made of such resins, liquid crystal polymers, polyamide, bismaleimide⋅triazine (BT) resins, etc. have too low viscosity, so that the thickness of the coated layer made of such resins can be a few μm thick at best for a single coating process, therefore the base of the support body is seen through the coated resin. Therefore, it is difficult to obtain sufficient reflection efficiency. When a large amount of resin is forcibly coated, resin tends to drip down, and volatilization of solvent occurs at the surface of the coated layer, so that wrinkles and unevenness in thickness are observed between the center and edge portions of the coated layer. Therefore, a plurality of coating and drying processes are carried out to form a white-colored reflecting layer having a desired reflectance and thickness (several dozens μm).
Alternatively, the reflective substrate has been formed by curing a non-silicone base high viscous varnish which contains reflective components.
In addition, the reflective substrates made of such resins or varnishes generally have a poor heat and light resistance. And yellowing is observed. The substrates have a tendency to absorb light having a wavelength of 400 nm or less, accordingly they cannot reflect light efficiently. Further, these reflective substrates made of such resins or varnishes can be produced easily and at a low cost, but in a re-flow process of a recent lead-free soldering, the substrates are heated up to about 300° C., initial degradation such as yellowing is observed. In addition, recently, the wavelength of emitting light has become shorter and emission power has become higher. Yellowing and degradation over time is seen under a high temperature caused by high-brightness white emission light. And the surface of the substrates become dull, and the decrease in reflective efficiency is also observed. Thereby, initially-designed illumination performance is degraded and their surface becomes dark, being inconvenient.
Versatile and simple reflective substrates, which can sufficiently reflect light having a wide wavelength range from a short wavelength of 340-500 nm to a longer wavelength such as the infrared region which are emitted from LED light sources, which can be used for wiring substrates and packages of solar cell assemblies as well as light emitting devices, and which show excellent heat and light resistance, excellent heat conductivity and light reflectance, no deterioration in light reflectance for a long period of time, is required.
Further, a simple manufacturing method for reflective substrates having reflective layers on support bodies having various shapes is also required. Further, a simple manufacturing method for reflective substrates having enough reflectance whose reflective layer can be made by single thick coating of reflective-layer base material compositions which can be formed into a film-like, 3-dimension-like or plate-like shape, is required. Further, a base material composition having a simple compounding recipe, which can be usable for forming a reflective layer having sufficient reflectance by a single thick coating of the base material composition and can be usable for reflective substrate, is also required.